The present invention relates to roller cone drill bits, and particularly to their sealing structures.
Oil wells and gas wells are drilled by a process of rotary drilling. In conventional vertical drilling (as shown in FIG. 4). a drill bit 10 is mounted on the end of a drill string (drill pipe plus drill collars), which may be miles long, while at the surface a rotary drive turns the drill string, including the bit at the bottom of the hole.
When the bit wears out or breaks during drilling, it must be brought up out of the hole. This requires a process called “tripping”: a heavy hoist pulls the entire drill string out of the hole, in stages of (for example) about ninety feet at a time. After each stage of lifting, one “stand” of pipe is unscrewed and laid aside for reassembly (while the weight of the drill string is temporarily supported by another mechanism). Since the total weight of the drill string may be hundreds of tons, and the length of the drill string may be many thousands of feet, this is not a trivial job. One trip can require tens of hours and is a significant expense in the drilling budget. To resume drilling the entire process must be reversed. Thus the bit's durability is very important, to minimize round trips for bit replacement during drilling.
Two main types of drill bits are in use, one being the roller cone bit. FIG. 3 shows an example of a complete bit (of the insert type), in which a set of rotary cones 302, each having many teeth or cutting inserts 304, are each mounted on rugged bearings on an arm 310. The bit's teeth must crush or cut rock, with the necessary forces supplied by the “weight on bit” (WOB) which presses the bit down into the rock, and by the torque applied at the rotary drive. While the WOB may in some cases be 100,000 pounds or more, the forces actually seen at the drill bit are not constant: the rock being cut may have harder and softer portions (and may break unevenly), and the drill string itself can oscillate in many different modes. Thus the drill bit must be able to operate for long periods under high and variable stresses in a remote environment.
As the drill bit rotates, the roller cones roll on the bottom of the hole. The weight-on-bit forces the downward pointing teeth of the rotating cones into the formation being drilled, applying a compressive stress which exceeds the yield stress of the formation, and thus inducing fractures. The resulting fragments are flushed away from the cutting face by a high flow of drilling fluid.
During drilling operations, drilling fluid, commonly referred to as “mud”, is pumped down through the drill string and out through the drill bit. The flow of the mud is one of the most important factors in the operation of the drill bit, serving both to remove the cuttings which are sheared from rock formations by the drill bit and also to cool the drill bit and teeth (as well as other functions). However, the fragments of rock in the mud (which are constantly being released at the cutting face) make the mud a very abrasive fluid.
FIG. 5 is a sectional view of the internal surfaces of a prior art cone; cylindrical surfaces 11 and 13 are two journal bearings, while bearing race 15 holds ball bearings which control the axial position of the cone. A seal gland 17 holds an elastomer seal.
At least one seal is normally designed into the arm/cone joint, to exclude the abrasive cuttings-laden mud from the bearings. When this seal fails, the abrasive cuttings-laden mud will very rapidly destroy the bearings. Thus the seal is a very critical factor in bit lifetime, and may indeed be the determining factor.
The special demands of sealing the bearings of roller cone bits are particularly difficult. The drill bit is operating in an environment where the turbulent flow of drilling fluid, which is loaded with particulates of crushed rock, is being driven by hundreds of pump horsepower. The flow of mud from the drill string may also carry entrained abrasive fines. The mechanical structure around the seal is normally designed to limit direct impingement of high-velocity fluid flows on the seal itself, but some abrasive particulates will inevitably migrate into the seal location. Particles of abrasive materials (fines and sediments) will tend to accumulate as an abrasive mass at the edge of the O-ring. (This phenomenon is referred to as “packing.”) This abrasive mass will abrade the O-ring-type seal, until it eventually reduces the sealing area of the O-ring seal and causes failure. Additional general information regarding seals can be found in Leonard J. Martini, PRACTICAL SEAL DESIGN, (1984) and in SEALS AND SEALING HANDBOOK (4.ed. M. Brown 1995), both of which are hereby incorporated by reference.
Some prior attempts have been made to reduce particulate incursion. Baker-Hughes bits are believed to have used a small mud wiper in combination with a small groove in the cone backface. Smith bits are believed to have used a “shale burn” insert which laterally diverts cutting pieces away from the dynamic crack.